Basement and cover rock history in western Tethys: HT-LP metamorphism associated with extensional rifting of Gondwana
Robert Hall
ABSTRACT: High-temperature-low-pressure metamorphism of the deep crust is probable during continental lithospheric extension. The 75-65 Ma cooling ages of metamorphic and magmatic rocks preserved in overthrust crystalline slices in the southern Aegean are most plausibly explained by late Cretaceous extension of the Apulian continental margin, rather than by subduction-related magmatism. Metamorphism and magmatism at depth can be correlated with those stratigraphic features of the cover sequences that indicate extension. In particular, the puzzling 'premier flysch' of the region is interpreted as one consequence of peripheral uplift associated with stretching.
Introduction
Many recent models for the evolution of passive continental margins imply the possibility of a metamorphic event recorded by the middle and lower crust, associated with subsidence and probably with lithospheric extension (Sleep 1971; Falvey 1974; McKenzie 1978; Royden et al. 1980; Sclater et al. 1980). The precise charac- ter of the metamorphic rocks formed during such an extensional event will depend on the mechanism(s) of crustal extension and the thermal history of the stretched region (Mid- dleton 1980), for example whether there is a heating event associated with extension, and also on the rate of sedimentation after extension and subsidence (Fig. 1). However, all stretching models imply certain metamorphic features. The rocks produced will be of a low-pressure facies series (Thompson and England 1984; Wickham and Oxburgh 1985) and the metamor- phic ages recorded by the rocks will be cooling ages which post-date the stretching event (Oxburgh 1982). The resulting metamorphic fabric could be a regional extensional foliation, a localized foliation associated with deep crustal shear zones, or a combination of the two. Asso- ciated folding may occur in zones of relative displacement, such as shear zones.
The metamorphic rocks formed during exten- sion of continental margins are beyond the reach of direct sampling; changes in the physical properties or fabric of the rocks may enable them to be recognized by geophysical methods (Falvey and Middleton 1981). However, they may be identified in former continental margins which have been deformed in mountain belts. Not only can such basement rocks be directly
observed in overthrust terrains, but evidence of the extensional history of the margin may also be discernible from interpretation of the sedimen- tary sequences deposited within the former con- tinental margin. If the basement and cover of a former continental margin can be reconstructed, then an extensional event may be recognizable which is recorded by metamorphic rocks of low- pressure facies, with cooling ages close to, but post-dating, a 'significant' event recorded stratigraphically by cover sediments. Strati- graphic indicators will vary with position in the margin, and will depend on global sea-level, but could include erosional unconformities, marked changes in sedimentation rates, and evidence of faulting where there is no evidence for compres- sional deformation. Basic igneous activity may occur if stretching proceeds to the point where the crust breaks. If more than one phase of extension occurs during the evolution of the continental margin the last extensional event will probably be the one recorded by the con- tinental basement; if the effects of stretching are uniformly distributed, then early events are likely to be overprinted by later events; although if the effects are heterogeneous then several stretching events could presumably be recorded and preserved by the basement rocks.
Late Cretaceous metamorphism in the southern Aegean
The southern Aegean represents part of the former Apulian continental margin (Bonneau 1982), which was extended in several stages during the Mesozoic. The compressional events, which formed the present orogenic belt linking
From AUDLEY-CHARLES, M. G. & HALLAM, A. (eds) Gondwana and Tethys Geological Society Special Publication No. 37, pp. 41-50.
42 R. H a l l
Mica Amphibole / I I ',
I \
Mica Amphibole/
A
I I I
Mica I Amphlib~ / ,' ' , K / , s I
' A \ /
, , \ I I
FIG. 1. Schematic pressure-temperature- time paths for rocks metamorphosed during rifting: (a) extension and cooling according to the model of McKenzie (1978), (b) exten- sion associated with magmatism, according to the model of Royden et al. (1980), (c) heating followed by erosional thinning of the crust, according to the model of Sleep (1971). The stability fields of the aluminium silicate polymorphs (A: andalusite, K:
Greece and Turkey, began in the early Tertiary (Aubouin et al. 1976; Jacobshagen et al. 1978); siliciclastic flysch sediments were deposited dur- ing the Palaeogene and orogenic deformation culminated in the Oligocene and early Miocene (Creutzburg and Seidel 1975; Bonneau 1982; Bonneau 1984). Most of the nappes found on the islands of the southern Aegean are composed of carbonate platform, slope, and basin lithologies representing the cover sequences of the extended continental margin, which were detached from their basement and stacked, in two principal deformation events in the late Paleogene (Hall et al. 1984). The structurally highest unit in the southern Aegean (Fig. 2) is sporadically preserved on the islands of Crete, Anafi, Nikouria, Donoussa and possibly Syros (Bonneau 1984) and has been described as a m~lange. It includes slices of metamorphic rocks and granitoids. The metamorphic rocks are of high-temperature-low-pressure facies, and include garnetiferous schists containing cor- dierite, andalusite and sillimanite; clinopyrox- ene amphibolites, meta-ultrabasics, calc- silicates and marbles (Bonneau 1972; Seidel et al. 1976, 1981; Diirr et al. 1978; Reinecke et al. 1982; Koepke and Seidel 1984). Metamorphic assemblages indicate maximum pressures of 4- 5 kbar at 650-700~ The metamorphic rocks are closely associated with I- and S-type granitoids and their contact hornfelses. Amphibole ages range from 79 to 66 Ma, whereas mica ages range 75-64 Ma. The intimate association of magmatic and contact metamorphic rocks in the same crystalline complex, and the systematically greater ages of amphiboles compared with micas, indicate that these are cooling ages. Using blocking temperatures of 500~ for amphiboles and 300~ for micas, and a linear cooling rate, indicates that the metamorphic maximum occurred before 78 Ma (Crete) and 72 Ma (Anafi). The beginning of this thermal event can only be estimated, but if the assump- tion is made that the heating rate to the metamorphic maximum was equal to the cooling rate, then the thermal event recorded began before at least 85 Ma (Crete) or 80 Ma (Anafi) (early Campanian or older).
Previous authors (Bonneau 1982; Reinecke et al. 1982) have argued that these late Cretaceous
kyanite, S: sillimanite) are shown for reference. The lines labelled mica and amphibole indicate approximate blocking temperatures for these minerals.
FI6.2. The southern Aegean region. Principal localities from which high-temperature-low-pressure metamorphic rocks of late Cretaceous age have been reported are shown in black.
metamorphic and magmatic rocks are remnants of a high-temperature belt, generated by sub- duction processes beneath part of the Pela- gonian realm. If they do record such processes, and the subsequent rapid uplift and cooling of an arc terrain as suggested, subduction must have begun before 85-80 Ma in order to subduct suf- ficient oceanic lithosphere to initiate melting�9 Reconstructions of the region vary considerably, and the continuation of the Pelagonian zone south-eastwards is still problematical, but all reconstructions now agree that any ocean basins that did exist in the Mesozoic were small (see Robertson and Dixon 1984 for review). In this case the effects of subduction of one of these ocean basins ought to be recorded over a wide area, and it is worth examining the late Creta- ceous stratigraphic record of the region for some indication of this event. Associated features expected might include evidence of compres- sional deformation and calc-alkaline volcanism and deposition of large volumes of volcaniclastic debris eroded from the uplifted arc terrain.
Late Cretaceous stratigraphy of the southern Aegean
The most notable feature of the deep-water basinal sequences in the region is a brief sili- ciclastic interval of early-late Cretaceous age which interrupted otherwise pelagic carbonate- chert sedimentation. This siliciclastic interval is well known in the Hellenides, where it is known as 'premier flysch' and is considered to be characteristic of the Pindos zone; the presence of greenstone detritus led Aubouin (1965) to link it with the ophiolite complexes of the internal Hellenides, which were emplaced at the begin- ning of the Cretaceous. For continental Greece and the Peloponnese, Fleury (1970) suggested that an east-west facies variation indicated transport of sediment axially along the Pindos Basin because he saw no evidence to support either an internal or external origin, but he was unable to determine whether transport was from the south or the north. Although the term premier flysch is often not used, the same sili- ciclastic interval can be recognized throughout
4 4 R. Hall
the southern Aegean, much further from the presumed source region, on Crete, Syrna, Tilos, Symi, and in south-west Turkey.
Throughout the Aegean, this siliciclastic interval displays a number of puzzling features. In continental Greece and the Peloponnese, the premier flysch is typically Cenomanian- Turonian in age, but may be locally as old as late Albian and as young as Senonian, with no obvious pattern to the beginning and end of siliciclastic sedimentation (Fleury 1980; Thiebault, 1982). In the southern Aegean, the age of this interval is similarly variable (Fig. 3); in west Crete it is dated as Cenomanian (Seidel 1971), in central Crete as Turonian (Bonneau and Fleury 1971, Hussin 1983), and further east as probably Turonian (Bernouilli et al. 1974) but locally possibly as old as Cenomanian (Roussos 1978). Considering its supposed axial origin, it shows surprisingly rapid thickness variations in the Peloponnese, is absent in some sections, and
most notably is not restricted to the deepest parts of the Pindos Basin; sections recorded by Thiebault (1982) show that it is present at the basin margins but not in the axial parts of the basin, where uninterrupted chert deposition continued throughout the early Cretaceous. The facies variations observed by Fleury (1970) have been confirmed elsewhere in the Peloponnese (Maillot 1973); a change from coarse calcareous sandstones in the west to shales and marls in the east is not easily explained by an axial transport model. Piper and Pe-Piper (1980) interpreted the west to east passage in the Peloponnese from coarse channelled to fine-grained lithologies as a proximal to distal transition, and palaeocurrent evidence indicating eastward transport led them to advocate an external (western) source region for the siliciclastic debris. They suggested that lack of siliciclastic sediments in the Tripolitza carbonate platform sequences of the same age to the west, the principal reason for Fleury's (1970)
~ basaltic volcanics
~ sandstones
~ shallow marine limestones
~ dolomites
A Ma
Eocene 50- I flysch
- Pa leocene
70" Late. --cM2 Cret . - - Sa ~ T C o I ~ 90- - - d ~ . ' 1
" AI 1 1 0 -
. Early - - 130-_~Cretaceous
150- Late ~---Z] . Jurassic ~ 1
170- Midd le
- Jurassic
190- _ Early [ ~
210- Jurass ic
230 - T r i ass ic
Middle Tr iass ic
~ calcarenites and calcirudites
calcarenites and calcisiltites ; ' ; ' , ' , ;
~ redeposited oolitic limestones
, ~ , pelagic limestones, cherts and shales ~ breccia
Ethia Series Xind0thi0 Series (4, 5) Central Crete Karpath0s (2,3) (4,5)
E F G
fysc• f l y s c h
|
I .... i , 1 1
i | i | i |
| i | i '
H I
. . . . . . \ -, i
i I
Ataviros Subgroup Rhodes (5)
Pindos Series Tilos Syrna Kokkimidi Group Archangelos Subgroup West Crete (6) (7) Symi Rhodes (1) (5) (5)
Fio. 3. Schematic stratigraphic sections through platform, slope, and basinal sequences in the southern Aegean. The early late Cretaceous extensional event is marked in different ways in platform sequences (D,/), slope sequences (B, F, G, H) and basinal sequences (A, C, E), principally by a brief siliciclastic interval and/or an unconformity. Based on information from: 1: Seidel (1971), 2: Bonneau and Fleury (1971), 3: Hussin (1983), 4: Davidson (1974), 5: Harbury (1986), 6: Roussos (1978), 7: Marnelis (1978). Abbreviations: Al Albian, Ce Cenomanian, Tu Turonian, Co Coniacian, Sa Santonian, Ca Campanian, Ma Maastrichtian. Time scale is that of Harland et al. 1982.
H i g h - t e m p e r a t u r e - l o w pressure m e t a m o r p h i s m 45
rejection of a western source region, could be explained by transport of siliciclastic sediments during times of lowered sea-level. They pointed to important disconformities in the Tripolitza limestone sequences in the early late Cretaceous in support of their hypothesis.
Many of the anomalous features of the premier flysch are also evident in the basinal sequences further east in the southern Aegean and Turkey (Fig. 3). In south-west Turkey, Syrna and Tilos, and other small islands of the southern Aegean, a coarse, angular, poorly- sorted breccia up to 10m thick containing Jur- assic and Cretaceous limestones, chert, and locally clasts of diabase, quartz, and mica (Bernouilli et al. 1974) marks the base of the siliciclastic interval. On Symi, breccias are com- mon in the lower part of the siliciclastic interval (Harbury 1986). These breccias have a very proximal aspect, which is odd if they are derived either axially from the northern Hellenides or externally from west of the Tripolitza platform; they are more plausibly interpreted as sub- marine fault-scarp breccias. The overlying flysch sequence consists of graded conglomerates, sandstones, and shales (Bernouilli et al. 1974; Harbury 1986), and a common feature is the presence of shallow-water carbonate debris and clasts of continental metamorphics, serpen- tinites and basaltic rocks; exactly the same type of debris as found to the west on Crete and in the Peloponnese. Volcanic clasts are typically basaltic, and calc-alkaline debris has not been found anywhere. Throughout the southern Aegean the sandstones are relatively coarse grained and not typically distal deposits. As in the Peloponnese, in most sequences there is a return to carbonate deposition in the Senonian, typically pelagic with redeposited beds, and although on Symi and in Turkey the siliciclastic interval is overlain directly by wildflysch of prob- able Palaeocene age, the presence of pelagic limestones of late Cretaceous age in the wild- flysch suggests the same return to carbonate deposition in the Senonian (Thorbecke 1976; Harbury 1986). Also as in the Peloponnese the siliciclastic interval is not identifiable in all basinal sequences; it is missing in the Profitas Ilias Subgroup of Rhodes, although it may be correlatable with Turonian marly limestones of the Xindothio Series on Karpathos (Davidson 1974). Although the siliciclastic interval may be absent, the deepest basinal sequences record renewed extension, and commonly include brec- cias composed entirely of basinal debris suggest- ing intrabasinal fault scarp deposits.
On Crete, blocks of pillow lavas (Bonneau 1973) are found in the same m~lange unit as the
metamorphic rocks. The lavas are MORB-like tholeiites (Robert and Bonneau 1982), indicat- ing significant partial melting of the underlying upper mantle and local breaking of the continen- tal margin crust. They are overlain by Senonian pelagic marls. On Crete, Karpathos, and Rhodes, tholeiitic dolerites and gabbros of early-late Cretaceous age (Mutti et al. 1970; Baranyi et al. 1975; Koepke et al. 1985) are exposed, which locally intrude recrystallized carbonates.
In shallower-water regions, carbonate deposi- tion continued throughout the late Cretaceous with no indication of arc volcanism. Areas that remained topographically high throughout the Mesozoic are represented by sequences of plat- form carbonates; at a time of rising global sea- level, parts of these platforms were uplifted slightly on Rhodes (Harbury 1986), Karpathos (Davidson 1974), and in the Peloponnese (Tsaila-Monopolis 1977) eroding up to a few hundred metres of the early Cretaceous car- bonates. Elsewhere the platform sequences record continuous sedimentation in shallow water on Crete (Leppig 1978), or subsidence to subtidal depths on Karpathos (Harbury 1986) on faults indicated by redeposited fault-scarp brec- cias, channelled calcirudites and calciturbidites. Carbonate slope regions which separated the platforms from basins, and had subsided earlier in the Mesozoic, record an increased input of redeposited shelf carbonate material into basinal areas, indicating local uplift at the basin margins. The siliciclastic interval is missing in the carbonate slope sequences of the region, but on Karpathos and Rhodes there is an abrupt change in sedimentation rate in the early late Cretaceous, and coarse redeposited carbonates of Senonian age rest directly on very early Creta- ceous limestones on Rhodes (Mutti et al. 1970; Harbury 1986), suggesting erosion and channel- ling. In some of the redeposited limestones, basaltic igneous and continental metamorphic lithic clasts, similar to those in the deeper basinal sequences, can be found (Hussin 1983).
Compression or extension?
The stratigraphy of the platform, slope, and basinal sequences of the southern Aegean and surrounding regions record evidence of a signifi- cant event in the early late Cretaceous. The basinal sequences are typically interrupted by a siliciclastic interval, which is unusual in their otherwise continuous record of carbonate and chert deposition. An unconformity can be
46 R. H a l l
recognized in many of the shallower-water car- bonate sequences. This is marked by erosion on parts of the platforms, subsidence of other parts of the platforms, and increased input of shallow- water debris on to the slopes flanking the basins. The siliciclastic interval has previously been interpreted as debris eroded and transported axially, from ophiolite nappes emplaced during the Eo-Hellenic orogenic phase in northern Greece. However this hypothesis fails to explain the variable thickness and timing of this interval, local facies variations such as those in the Peloponnese, and the very proximal character of the deposits in areas increasingly far from the supposed original source. It is odd that the Eo- Hellenic mountains, formed at the beginning of the early Cretaceous, should have revived suffi- ciently to provide debris to the southern Aegean and Turkey, when intervening regions such as the Argolis Peninsula (Bachmann and Risch 1978) had long since received the products of their erosion, and carbonate deposition had resumed. A very complex system of sediment erosion, transport, and distribution is required for this explanation to be satisfactory. Piper and Pe-Piper (1980), who drew attention to some of the anomalous features of the premier flysch interval in the Peloponnese, suggested a western source for siliciclastic sediment, which explains local facies variations but is less attractive when such sediment has to be transported as far east as Turkey. Their proposal that intervals of lowered sea-level allowed channelling of siliciclastic debris across a carbonate platform, is also incon- sistent with evidence of a global rise in sea-level in the late Cretaceous after a possible mid- Cretaceous lowering of sea-level (Vail et al. 1977; Hallam 1984; Watts and Thorne 1984). In none of the sequences of the region is there evidence for a late Cretaceous episode of compression.
A simpler and more satisfactory explanation is the proposal that the stratigraphy of the region indicates an important episode of extension in the early late Cretaceous. Eastwards of the southern Aegean, the abundant ophiolites are of early-late Cretaceous age or younger. Geochemical arguments have been advanced to suggest that these ophiolites are subduction- related, although there is little stratigraphic or sedimentological support for such an interpreta- tion (Hall 1984; Woodcock and Robertson 1984). I suggest that it was late Cretaceous extension which led to important and wide- spread ophiolite formation east of Greece, and stratigraphic evidence of this extensional epi- sode is to be expected in adjacent parts of the southern Tethyan margins. Intermittent fault-
ing, over a several million year period early in the late Cretaceous, could account for the vari- ation in age of the siliciclastic interval. Active extensional faults shed debris, which accounts for the locally very proximal character of breccia deposits and also their rapid changes in thick- ness. Uplift at the basin margins caused erosion of platform and upper slope sequences, in some cases eroding through the carbonate cover to the underlying metamorphic basement. Channel- ling of this debris from adjacent platform regions explains local facies variations which indicate external source regions. Piper and Pe-Piper (1980) note the similarity of the heavy-mineral assemblages in the premier flysch of the Peloponnese to material from the Phyllite Series, and such a similarity is probable if, as suggested by several authors, the Phyllite Series of the Peloponnese and Crete is the likely base- ment of the Tripolitza and other platform car- bonates of the region. Local uplift of platform regions at basin margins could be a consequence of extension of the platform carbonates on listric faults, allowing subsidence and local uplift of rotated parts of these blocks. An alternative explanation could be local peripheral uplift of thermal origin at basin margins, as predicted by several extensional models (Sclater et al. 1980; Royden and Keen 1980; Beaumont et al. 1982). Faults associated with extension provided access for basic extrusives, and account for the intru- sion of basic rocks into carbonates seen on Crete, Karpathos, and Rhodes. Erosion of the basic igneous rocks which reached the surface, and the basement underlying the platform car- bonates in channels at the basin margins, pro- vided siliciclastic debris for the flysch sediments. In the deeper parts of some basins, faulting formed intraformational breccias in the basinal sequences, and locally, stretching proceeded to the point where the continental crust broke and true oceanic crust may have formed (e.g. the Arvi ocean of Bonneau (1984) on Crete). Thermal subsidence and rising sea-level restored carbonate deposition to all areas by the Campanian (?80 Ma).
Metamorphism associated with extension
There is little evidence for subduction and calc- alkaline volcanism in adjacent regions in the early late Cretaceous. No calc-alkaline volcan- ism is recorded, no calc-alkaline volcanic debris appears in the sedimentary sequences, and there is no evidence of compressional deformation in the region. Radiometric dating of high-P-low-T
H i g h - t e m p e r a t u r e - l o w
metamorphic rocks in the region indicates Jur- assic, early Cretaceous and Tertiary ages, but no late Cretaceous ages (Diirr etal. 1978; Maluski et al. 1981). The absence of evidence for subduc- tion and volcanism could be explained by sup- posing that all of the ocean basins were so small that they were subducted with little or no trace of the normal subduction processes. However, this requires special pleading for practically every postulated ocean in the region, and also an unusually prolonged period of very slow subduc- tion of very small oceans. Instead of a subduc- tion-related origin for the late Cretaceous igneous and metamorphic rocks, I suggest the alternative interpretation that the metamorphic and magmatic rocks record a thermal event associated with lithospheric extension within the Apulian continental margin. The mineral ages represent, not uplift and cooling, but subsidence and cooling after heating and extension of the margin early in the late Cretaceous (?95-90 Ma; Cenomanian-Turonian). The mineral ages record the point in the late Cretaceous where the extended and heated basement cooled to appropriate blocking temperatures.
The southern Aegean is unusual, in that con- tiental-margin sedimentary cover sequences are still well preserved, while overthrust basement rocks have not yet been entirely eroded off the uplifted orogenic belt. The basement rocks were thrust over their cover in the late Paleogene, to a structurally high position, at an early stage in the compression of the continental margin, and thus escaped the effects of the regional metamor- phism associated with collision. It is therefore possible to compare the basement and cover histories, which is not possible in undeformed passive margins or uplifted continental rift zones, where either cover-only or basement- only is seen. The deeper basement rocks of the region are generally not exposed, or where exposed are overprinted by Alpine metamor- phism, and therefore the regional extent of this metamorphic event is uncertain. However, dis- covery of rudists in marbles forming the cover to the Menderes Massif of western Turkey indicate that it formed the basement of a Mesozoic plat- form sequence, and although overprinted by Alpine metamorphism, one 66 + 4 Ma muscovite age is reported (Diirr et al. 1978). Diirr et al. regard this age as somewhat high for Alpine metamorphism, but the position of the Menderes Massif on the edge of the southern Aegean, and its reported high-T-low-P andalusite-bearing rocks (Evirgen and Ataman 1981) give rise to the speculation that the late Cretaceous metamorphic event was more wide- spread than its preserved relics now suggest.
pressure m e t a m o r p h i s m
Conclusions
47
There appears to be little stratigraphic support for a subduction-related and compressional origin of the metamorphic event recorded by continental basement metamorphic and mag- matic rocks. In contrast, the interpretation of these rocks as a product of lithospheric exten- sion is consistent with the regional stratigraphy, with the recent suggestion that continental rifts are a probable location of high-temperature- low-pressure metamorphism (Wickham and Oxburgh 1985), and with suggestions that many of the deep basins within the southern Tethyan margin were narrow (Jenkyns and Winterer 1982) and underlain largely by continental crust (D'Argenio and Alvarez 1980; Hall 1982, 1984). Not until the late Cretaceous extensional phase did many of these deep basins within the con- tinental margin east of the south-east Aegean acquire an oceanic foundation as the Tethyan margins were stretched for the last time. Exten- sion may have begun as early as the Albian, and may have continued until the early Senonian; in the south-east Aegean it seems to have been most important in the Turonian. Small ocean basins developed (probably largely within the continental margin of the southern Tethys) dur- ing this final extensional phase, and closed later in the Cretaceous, finally emplacing the oceanic crust as ophiolites. In mainland Greece where small ocean basins had opened and closed much earlier--by the earliest Cretaceous, the exten- sion further east was recorded by the premier flysch. The premier flysch, which is widespread in the Hellenides and recognizable throughout the southern Aegean and in south-west Turkey, and local unconformities in the platform and slope sequences, may be the only record of regional extension in areas where Alpine meta- morphism has overprinted earlier events in the basement.
High-T-low-P metamorphic rocks associated with crustal extension will be observable in oro- genic belts where collision processes have thrust basement rocks over their former cover sequences. However, to escape regional meta- morphism associated with orogenesis, they must be emplaced at high structural levels in the mountain belt, and therefore such rocks will be exposed only for a short period after collision, before they are eroded away. In the Aegean, post-orogenic extension and subsidence during the Neogene has resulted in the preservation of remnants of the higher thrust slices, but elsewhere only the youngest orogenic belts will expose such rocks. One such example is in
48 R. H a l l
Timor, where collision between the Australian continental margin and south-east Asia began only in the last 3.5 Ma, resulting in overthrusting of basement rocks of the former rifted margin on to the Australian margin sedimentary cover rocks (Audley-Charles 1981). High-T-low-P metamorphic rocks with decompressional P -T paths have been described from Timor by Brown and Earle (1983), and like the Aegean rocks have been attributed to metamorphism associ-
ated with crustal rifting. Such extremely young orogenic belts may offer the best chance to relate the stratigraphic features of extension to the thermal processes occurring during crustal extension.
ACKNOWLEDGEMENTS: Fieldwork in the Aegean was funded by NERC and the Central Research Fund of London University. I thank Neil Harbury for help and discussion.
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ROBERT HALL, Department of Geological Sciences, University College London, Gower Street, London WCIE 6BT, UK.
